1. Field of the Invention
The present invention relates to a technology for the fault diagnosis of an electronic device including an image forming apparatus.
2. Description of the Related Art
Synchronous data transmission (uni-directional) is known and can be faster relative to asynchronous data transmission (bi-directional). Japanese Patent Laid-Open No. 2002-230536 discloses a dedicated data transfer method, by which the data transfer is fixedly in the one-way direction. This has the advantage that the serial bus reduces the number of signal lines and the physical connection between the devices becomes easy. Also, since the data transfer is in the one-way direction, the timing of an interface circuit is better designed.
Further, Japanese Patent Laid-Open No. 4-100446 discloses a data transfer system, wherein a network is switched to a backup network and a faulty device is disconnected when a packet error is detected. The disadvantage is that for an image forming apparatus, one portion of the image forming apparatus is disconnected and is not used.
Furthermore, Japanese Patent Laid-Open No. 02-153655 discloses use of a loop-back communication test in a communication control integrated circuit to determine whether the communication control integrated circuit has a fault. To specify the fault portion, the loop-back communication test is needed for an internal portion as well as for an external portion, which results in complicated control circuitry. In an apparatus which does not communicate with the external portion, it is too difficult to specify the fault portion.
As mentioned above, synchronous communication is important for improving the speed of data transfer. When this data transfer mode is used, and devices are daisy chained in loop, access to all the devices is cut off when a fault occurs. Therefore, a user can find only the existence of fault and, inconveniently, the extensive test using a dedicated measuring instrument is necessary for identifying the faulty portion.
A boundary scanning method is popular for fault diagnosis of devices in factories. However, the boundary scanning method is not suited to the self-diagnosis of devices for maintenance in the field. That is, the boundary scanning method needs a one-to-one correspondence between test data and the device and, upon changing the device, use of the test data must be changed accordingly. In self-diagnosis, the version of the device is checked and the test data suitable thereto is selected. However, since the device information is not readable because access to the device has been cut off, other ways such as using previously stored information is used. Since test data for boundary scanning is large (approximately 100 KB per LSI) pre-installing the test data is not cost effective.
Therefore, one advantage of the present invention is that it provides an electronic device in which, when a plurality of processors are connected in a loop, for example, by a one-way bus, a faulty portion in the structure is easily detected.